Beam forming has been studied in cellular applications for many years but has not been widely adopted in commercial systems so far. Now the situation appears to change and one example where beam forming is beginning to be commercially used is in TD-SCDMA in China. Further, LTE standardization seems to rely on beam forming as one basic component.
It is common to distinguish between two types of beam forming; a closed loop beam forming that is based on information obtained by the receiving part being fed back to the transmitting part, and a non-closed loop beam forming that is based on information determined from a signal received without a priori knowledge of the transmitting part.
In the closed loop beam forming, measurements are performed at the intended receiver of the transmission link targeting “optimization”, and the measurements/metrics are fed back to the transmitter via a return transmission channel. The feedback measurements/metrics are planned to be used in the subsequent transmissions to “optimize” the transmission of a signal to the intended receiver that performed the measurements. The procedure implies an a priori defined sequence or protocol suite to be utilized. A detailed description of closed loop beam forming is presented in connection with FIG. 1a. 
Examples are closed loop TX diversity in WCDMA and codebook based precoding in LTE, reference [1] chapter “6.4 Precoding” for up to 4 layers.
In the non-closed loop beam forming, measurements are performed on a received signal without a priori knowledge of the transmitting part, and the measurements/metrics are used for “optimizing” the transmission on a return channel, i.e. transmission from the measuring node to the previous transmitting party as receiver. This implies that measurements may be performed by the measuring node on any signal generated by the intended receiver to “optimize” transmission on the return channel even if the measurements are performed on a signal intended for any other node. A detailed description of non-closed loop beam forming is presented in connection with FIG. 1b. 
A foundation for non-closed loop beam forming in LTE is given by the introduction of UE-specific reference signals as described in reference [1], chapter 6.10.3.
The published international application WO 01/69814 A1, reference [3], assigned to Nokia Networks OY, discloses a system that uses a closed loop a priori defined protocol suite according to the definition above for the used measurements. Specifically it is described according to the prior art methods defined in the 3GPP WCDMA standard for closed-loop Tx-diversity utilizing the available pilot signal structures. Described is also that this closed loop modes in a similar fashion may be extended to multiple-antennas beyond the two antenna method defined in 3GPP. Further, this closed-loop mode feedback is however the only measurements utilized in their “optimization” of the transmission channel, no additional measurements are utilized in that process.
A drawback with channel estimation in the prior art is that the channel estimation will be challenging at low levels of received SNR/SNIR, and further as a consequence creating less accurate momentarily preferred Tx-diversity/pre-coding antenna weight vectors jeopardizing the performance of multi-antenna modes.